Cold storage systems have been proposed for various purposes. In building air conditioning systems, cold storage systems have been proposed for shifting power consumption from peak demand periods to off-peak periods. The cold storage medium is incorporated into an air conditioning system and cooled by operation of the air conditioning system during off-peak hours. The cold storage medium, itself, is then used to cool the building during peak hours.
It is also been proposed to incorporate cold storage systems into automotive air conditioning systems. As with the building systems, the cold storage medium is disposed in the air conditioning system, and is cooled during operation of the air conditioning system. The cold storage medium, itself, is then used to cool the interior of the vehicle when power demand on the vehicle engine is high, as for example, when the vehicle is climbing a hill, or when the vehicle is parked with the engine not running.
Various types of media have been proposed as cold storage media including water and/or ice, brine and the like. Particularly desirable as cold storage media are those systems which change phases between solid, liquid and/or gas thereby employing the phase transition energy for thermal storage. These systems include pure compounds, such as formic acid, inorganic hydrates or eutectic inorganic hydrates, such as for example, sodium chloride--sodium sulphate decahydrate; and the like.
Particularly desirable phase change thermal storage media for commercial air conditioning systems are those materials known as gas hydrates disclosed in, for example, U.S. Pat. No. 4,540,501 to Ternes et al. Gas hydrates are non-stoichiometric crystalline solids classed as clathrate compounds. More particularly, gas hydrates are solid crystalline structures with the gas molecules trapped within the ice-type lattice. The trapped gas lends stability to the structure, which permits most such hydrates, which are principally water, to exist as a solid at temperatures well above the 32.degree. F. freezing point of water. The melting or reversible decomposition of such gas hydrates in a sealed tube requires a heat input of approximately 120 Btu/lb. This is similar to the cold storage capacity of ice/water transition (144 Btu/lb); but, with gas hydrates, this capacity is deliverable at temperatures within the range of conventional air conditioning systems. Despite these advantages, actual use of gas hydrates as cold storage media has been discouraged by the fact that formation of the hydrate requires temperatures much lower than its transition temperature, particularly when all of the hydrate has been decomposed and none remains in the medium.
A thermal storage gas hydrate system which provides for advantageous hydrate formation at temperatures at or only slight below the gas hydrate transition temperature is disclosed in U.S. Pat. No. 4,922,998 to Carr, issued May 8, 1990. In this system, a movable mechanical device is mounted for free movement within a container containing the gas hydrate and thereby facilitates hydrate formation at more desirable temperatures.
In the known cold storage air conditioning systems, the cooling of the cold storage medium is often achieved by indirect heat exchange between the cold storage medium and the air conditioning system. For example, air which has been cooled by the air conditioning system cools the cold storage medium by indirect heat exchange.
In the commercial air conditioning systems, including vehicle and building air conditioning systems which have been proposed for use with cold storage media, a proportion of the cooling load is used to condense water out of input air as it is cooled. This water is typically discarded as a waste product of the operation, although in some air conditioning systems, such as that disclosed in U.S. Pat. No. 4,406,138 to Nelson, the condensed water is used to enhance the efficiency of the air conditioning condensing unit by spraying of the condensed water onto the condensing coils.
U.S. Pat. No. 4,018,060 to Kinsell et al proposes a system for improving the efficiency of an aircraft air conditioning system. According to this disclosure, ambient air is drawn into a turbine for expansion and cooling. The air passes through a heat exchanger where it withdraws heat from recirculated cabin air. Water can be sprayed into the low pressure air at the heat exchanger to provide additional cooling by evaporation.
U.S. Pat. No. 4,440,698 to Bloomer discloses an apparatus for ensuring heat exchange between a gas flow and a heat exchanger and which is proposed for use in recovery of heating or cooling energy from industrial process exhaust, such as for the recovery of energy from air exhausted from an air conditioned building. According to this proposal, the heat exchanger is mounted in a duct with constrictions for the gas flow arranged in the duct such that jets of gas are created. Liquid is sprayed within the duct and the gas jets pick up the liquid and carry it onto the heat exchanger surface thereby thoroughly wetting the heat exchanger. When the heat exchanger contains a medium to be cooled, evaporation of the liquid on the heat exchanger absorbs heat from the heat exchanger coils.
Energy efficiency improvements are particularly desirable in air conditioning systems which include cold storage medium because cooling of the cold storage medium can add substantially to the cooling load on the air conditioning system. Moreover, when the phase transition type cold storage media are employed, the phase transition temperature of the media may be close to the temperature of the air conditioned air and a substantial amount of heat exchange can therefore be required in order to effect phase transition of the cold storage media.